Evidence regarding the effectiveness of after-school programs (ASPs) for reducing problem behaviors is mixed. Unstructured ASPs may increase antisocial behavior by increasing " deviancy training" opportunities, when peers reinforce deviant attitudes and behaviors. This research analyses approximately 3000 five-minute intervals from 398 observations of activities delivered as part of an ASP in five public middle schools. Analyses of peer and group leader responses to deviant behavior in the context of ASP activities indicated that while peer responses are generally reinforcing, group leaders typically do not respond to deviance. Multi-level analyses of the association between activity structure and deviant behavior indicate that higher levels of structure in the activity as a whole decrease levels of violence and counternormative behavior. As the level of structure in five-minute intervals within the activity increases, the level of violent behavior declines, but violent talk (e.g., threats to commit violence) increases. Implications for after-school programming are discussed. © 2010 The Association for Professionals in Services for Adolescents.